New design of GaAs based THz-QCL for obtaining high opticalgain by indirect-injecting asymmetric-wells superlattice structure

摘要

THz-QCLs has been attracted huge attentions sincetheir first realization. However, the operatingtemperature until now is still below 200K [1]. It isquite significant to find new device designs toachieve room temperature lasing, at least, to be upto the thermoelectric cooling 230K. Compared tothe designs using resonant tunneling (RT) and LOphonon depopulation which realizes the worldrecord operating temperature of 199.5K (Fig.1a),we proposed a new design and simulated it byNEGF method: 1. It is the GaAs/AlGaAssuperlattice with asymmetric wells. The upperlasing well is Al0.06Ga0.94As. The lower lasingwell is GaAs and very narrow to avoid themulti-periods current leakage raised from the highenergy levels in this well (Fig.1b); 2. Injectingprocess is realized by the inter-well LO phononscattering in the upper lasing well (also we call itas indirect injecting process). 3. One period onlyconsists of two wells and the efficient extraction €of electrons from lower lasing levels (l) is byresonant tunneling. In Fig.2c, it is clear that in thisnew design the electrons mainly stay at the upperlasing levels (u), which results in a betterpopulation inversion (u→l). Such electronsdistribution is also benefit to avoid the formationof negative differential conductivity (NDC) whichcan make the electric field inhomogeneous. Ascompared, in the world-record design with RT andLO phonon depopulation (Fig.2d), many electronsare retarded at injecting level(i). From the I-V andoptical gain in Fig. 3, it can be found that the newdesign results in an improved maximum gain of74cm-1 at the operating bias per period (54mV)where the world-record design is 59cm-1, and itsthreshold current is also quite small (∽700A/cm2)and also the lasing dynamic range is much larger(700→1305A/cm2). The corresponding values ofthe world record designs are 1060A/cm2 and1060→1200A/cm2, respectively.